This is a follow up to cockroachdb#79725. Previously, there were two issues:
1. Some of the ConnTracker APIs (e.g. GetConns and OnDisconnect) were forcing
   creation of tenant entries even though that was unnecessary.
2. We were holding onto a large portion of memory (with connections from all
   tenants).

This commit addresses the two issues above: (1) tenant entries are no longer
being created when that was unnecessary, and (2) instead of holding onto all
the connections at once, we will retrieve the tenant's connections one by one.
At the same time, to avoid extra allocations that are solely used during data
transformations, we update the ConnTracker API to return a list of connection
handles indexed by pod's address directly.

Note that we may reintroduce the data transformation in future PRs, depending
on how we implement the connection rebalancing logic. That is currently
unclear.

Release note: None

This is a follow up to cockroachdb#79725. Previously, there were two issues:
1. Some of the ConnTracker APIs (e.g. GetConns and OnDisconnect) were forcing
   creation of tenant entries even though that was unnecessary.
2. We were holding onto a large portion of memory (with connections from all
   tenants).

This commit addresses the two issues above: (1) tenant entries are no longer
being created when that was unnecessary, and (2) instead of holding onto all
the connections at once, we will retrieve the tenant's connections one by one.
At the same time, to avoid extra allocations that are solely used during data
transformations, we update the ConnTracker API to return a list of connection
handles indexed by pod's address directly.

Note that we may reintroduce the data transformation in future PRs, depending
on how we implement the connection rebalancing logic. That is currently
unclear.

Release note: None

79477: ccl/sqlproxyccl: add server name indication (SNI) support r=darinpp a=darinpp

Previously the proxy supported two ways of providing tenant id and
cluster name information. One was through the database name. The second
was through the options parameter. As part of the serverless routing
changes, we want to support routing with a tenant id passed through SNI.
This PR adds the ability to route using SNI info.

Release justification: Low risk, high reward changes to existing functionality

Release note: None

79629: clisqlshell: implement `COPY ... FROM STDIN` for CLI r=otan a=otan

Resolves #16392

Steps:
* Add a lower level API to lib/pq for use.
* Add some abstraction boundary breakers in `clisqlclient` that allow a
  lower level handling of the COPY protocol.
* Altered the state machine in `clisqlshell` to account for copy.

Release note (cli change): COPY ... FROM STDIN now works from the
cockroach CLI. It is not supported inside transactions.



79677: sql: session variable to allow multiple modification subqueries of table r=rytaft a=michae2

Add a new session variable, enable_multiple_modifications_of_table,
which can be used instead of sql.multiple_modifications_of_table.enabled
to allow execution of statements with multiple modification subqueries
of the same table.

Instead of making the original cluster setting the GlobalDefault of this
new session setting, the original cluster setting is kept in the
optbuilder logic. This is to avoid breaking applications that are
already toggling the cluster setting mid-session to allow statements.

Fixes: #76261

Release note (sql change): Add a new session variable,
enable_multiple_modifications_of_table, which can be used instead of
cluster variable sql.multiple_modifications_of_table.enabled to allow
statements containing multiple INSERT ON CONFLICT, UPSERT, UPDATE, or
DELETE subqueries modifying the same table. Note that underlying issue
70731 is not fixed. As with sql.multiple_modifications_of_table.enabled,
be warned that with this session variable enabled there is nothing to
prevent the table corruption seen in issue 70731 from occuring if the
same row is modified multiple times by different subqueries of a single
statment. It's best to rewrite these statements, but the session
variable is provided as an aid if this is not possible.

79697: sql: ensure descriptors versions change only once, fix related bugs r=ajwerner a=ajwerner

The first commit adds back the change from #79580.

The second commit fixes fallout. As soon as #79580 merged, it tickled flakes. These flakes were caused by
operations to alter the database which would build new mutable tables from
immutable tables and thus overwrite existing entries. The fix here is to
retrieve the proper descriptors using the collection, and to make sure
that those descriptors are properly hydrated. This, in turn, revealed that
our policy checking in validation to avoid using descriptors which had been
already checked out was too severe and would cause excessive numbers of extra
round-trip.

I'll be honest, I haven't fully internalized why that policy was there. I
supposed it was there to ensure that we didn't have a case where we check
out a descriptor and then fail to write it to the store. I don't exactly
know what to do to re-establish the desired behavior of the code. At the
very least, it feels like if we did re-read the descriptor once, then we
should do something about resetting the status. I guess I could try to
unravel the mystery leading to the checkout in the first place.

The test is very flakey without this patch.

The third commit sets `AvoidLeased` flags which ought to be set. It's sad
that they ought to be set and that it's possible that there were bugs due to
the flag not being set.

Backport will address #79704.
 
Release note: None

79857: ccl/sqlproxyccl: cleanup balancer and connection tracker r=JeffSwenson a=jaylim-crl

This is a follow up to #79725. Previously, there were two issues:
1. Some of the ConnTracker APIs (e.g. GetConns and OnDisconnect) were forcing
   creation of tenant entries even though that was unnecessary.
2. We were holding onto a large portion of memory (with connections from all
   tenants).

This commit addresses the two issues above: (1) tenant entries are no longer
being created when that was unnecessary, and (2) instead of holding onto all
the connections at once, we will retrieve the tenant's connections one by one.
At the same time, to avoid extra allocations that are solely used during data
transformations, we update the ConnTracker API to return a list of connection
handles indexed by pod's address directly.

Note that we may reintroduce the data transformation in future PRs, depending
on how we implement the connection rebalancing logic. That is currently
unclear.

Release note: None

79874: ccl/sqlproxyccl: add metrics to track the number of rebalance requests r=JeffSwenson a=jaylim-crl

This commit addresses a TODO by adding metrics to track the number of rebalance
requests (total, queued, and running). The following metrics are added:
- proxy.balancer.rebalance.running
- proxy.balancer.rebalance.queued
- proxy.balancer.rebalance.total

Release note: None

Co-authored-by: Darin Peshev <darinp@gmail.com>
Co-authored-by: Oliver Tan <otan@cockroachlabs.com>
Co-authored-by: Michael Erickson <michae2@cockroachlabs.com>
Co-authored-by: Andrew Werner <awerner32@gmail.com>
Co-authored-by: Andrew Werner <ajwerner@cockroachlabs.com>
Co-authored-by: Jay <jay@cockroachlabs.com>

This is a follow up to cockroachdb#79725. Previously, there were two issues:
1. Some of the ConnTracker APIs (e.g. GetConns and OnDisconnect) were forcing
   creation of tenant entries even though that was unnecessary.
2. We were holding onto a large portion of memory (with connections from all
   tenants).

This commit addresses the two issues above: (1) tenant entries are no longer
being created when that was unnecessary, and (2) instead of holding onto all
the connections at once, we will retrieve the tenant's connections one by one.
At the same time, to avoid extra allocations that are solely used during data
transformations, we update the ConnTracker API to return a list of connection
handles indexed by pod's address directly.

Note that we may reintroduce the data transformation in future PRs, depending
on how we implement the connection rebalancing logic. That is currently
unclear.

Release note: None